This invention pertains to the volumetric measurement of flow and, particularly, a compact flow prover useful in periodically calibrating a continuous flowmeter in a pipeline without interrupting the flow of fluid therethrough. The compact flow prover of this invention falls generally into that class of flow provers characterized by the measurement of the movement of a piston traveling through a cylinder. This invention pertains specifically to a compact flow prover having the improved qualities of accuracy, dependability, infrequent and simplified maintenance, simple and light weight construction, low space requirements, and operating flexibility.
One type of device commonly employed for determining the accuracy of a continuous flowmeter is known as a calibrating loop. This device typically comprises a long run of pipe through which a free moving plug or sphere is propelled by the fluid moving therethrough. By measuring the time that it takes the object to move from one detector switch to another, the rate of fluid flowing through the flow loop can be determined. This type of device is the subject matter of U.S. Pat. No. 2,948,142 to Zimmerman, U.S. Pat. No. 2,948,143 to Pruitt, U.S. Pat. Nos. 3,423,988 and 3,668,923 to Grove, and U.S. Pat. No. 3,530,705 to Lathrop. Calibrating loops generally require a substantial length to maintain a usable accuracy. The high cost of these devices prevents their use in all but the most critical situations.
The use of positive displacement piston-type flowmeters is well documented. Examples of this type of flowmeter which are not particularly suited for calibration purposes include U.S. Pat. No. 1,586,834 to Ormsby, U.S. Pat. No. 2,652,953 to Gray, U.S. Pat. No. 2,892,346 to Sargent, and U.S. Pat. No. 4,096,747 to Gilson.
An example of a positive displacement piston-type flow prover useful in calibrating flowmeters is seen in U.S. Pat. No. 3,021,703 to Pfrehm which discloses a bidirectional, free-moving piston in a calibration barrel. The movement of the piston is detected by two detector switches which are mechanical in nature, located toward either end of the calibrating barrel. In order to obtain usable accuracy, a considerable run of pipe is required. For example, if the accuracy of the detector switch is .+-.0.05 inches, a run of pipe of 42 feet or more is required to yield desired accuracy of .+-.0.02%. In addition, since the detector switch protrudes into the calibrating barrel and the outlet ports are connected to the calibrating barrel, the piston seals will be subject to wear each time they pass the switches and the outlet ports. Because valves must be opened and closed simultaneously, further inaccuracies may be introduced into the measurement and serious disruption of flow may occur.
A device similar to U.S. Pat. No. 3,021,703 is disclosed in U.S. Pat. No. 3,580,045, also to Pfrehm. The device differs from U.S. Pat. No. 3,021,703 in that the mechanical detector switches are replaced with external proximity switches which detect the passage of a steel band on the piston. The use of a four way spool valve which reduces the complexity of the valving arrangement is also disclosed. However, it is believed that the seals of this valve are subject to wear and leakage and are not amenable to monitoring of seal integrity.
U.S. Pat. No. 3,273,375 to Howe discloses another type of flow prover comprised of inner and outer tubular members and a free-moving piston located in the inner tubular member. This arrangement allows for a simpler valving system and eliminates the need for pressure correction. However, the device uses the same external proximity switches discussed above and it is believed that the piston seal life is not greatly improved because the piston must move past outlet ports. In addition, special arresting means are required to stop the movement of the piston at either end of the calibrating barrel. Moreover, it is believed the complicated construction makes assembly and maintenance difficult.
U.S. Pat. Nos. 3,492,856 and 4,152,922, both to Francisco, disclose flow provers in which outlet ports on the calibrating barrel are not necessary. These devices utilize a flow-through piston having a poppet valve in the piston which may be closed during the proving cycle. The poppet valve requires pressuring means such as gas to close the poppet valve at the beginning of the proving cycle. U.S. Pat. No. 3,492,856 discloses external proximity switches located on the calibration barrel and retrieves the piston at the end of the proving cycle by means of a cable and drum.
U.S. Pat. No. 4,152,922 discloses a piston retracting means comprised of a rod connected to a measuring piston which has at its other end a retracting piston located in a hydraulic cylinder. The movement of the measuring piston is detected by proximity switches which detect the movement of the retracting piston in the hydraulic cylinder. In this prover, the measuring piston seals may be damaged by the presence of entrained solids such as grit or sand which may be in the measured fluid and become trapped between the calibrating barrel and the piston seals. If this prover is operated in a horizontal position the situation is aggravated since the solid particles may settle along the bottom of the cylinder.
A further problem associated with the flow provers heretofore mentioned is that horizontal operation results in reduced piston seal life along the bottom of the piston because of the weight of the piston.
It is a feature of this invention to provide a compact flow prover which does not have the disadvantages associated with the devices heretofore known. A further feature is to provide a compact flow prover with improved accuracy and dependability, reduced space and weight requirements, simplified construction and maintenance, and operating flexibility. Still another feature is to provide a compact flow prover with the aforementioned features which is operable at high pressures. These and other features will be apparent to those skilled in the art in the following description of the invention.